EVIDENCE FOR SHALLOW-WATER ORIGIN OF A DEVONIAN BLACK SHALE, CLEVELAND SHALE MEMBER (OHIO SHALE), NORTHEASTERN OHIO

There has long been a debate about whether the Devonian Cleveland Shale Member (Ohio Shale) was deposited in shallow- or deep-water depositional environments. This study looked at the Cleveland Shale Member (CSM) at 3 stratigraphic sections and 5 well cores from four counties in northeastern Ohio. The CSM mostly consists of interbedded dark gray and light gray carbonaceous fissile mudstones or claystones. The dark gray shale (mean thickness 13 cm) and light gray shale (mean thickness 17 cm) are rhythmically bedded and represent changes in carbon content. Three types of event layers are interbedded with the mudrocks: (1) tempestites (hummocky stratified fine-grained sandstones overlain by planar laminated very fine-grained sandstones overlain by ripple laminated or climbing-ripple laminated very fine-grained sandstones); (2) distal turbidites (normally graded normally graded fine-grained sandstones, siltstones, and shales); and (3) hyperpycnites (normally graded microlaminae of very fine-grained sandstone, siltstone, and shale). The tempestites overlie sole marks such as groove casts, and these indicate transport directions NE-SW (n = 56 measurements). The tempestites average 4.9 cm thick, are common at the base of the unit, and contain a newly observed trace fossil (Neonereites). The turbidites average 6.2 cm thick and are more common at the top of the unit. The hyperpycnites have been studied primarily using petrography and the SEM. Hyperpycnites are common throughout the CSM, for example at least 10 individual hyperpycnites were identified within a single stratigraphic interval ~ 1.73 cm thick. The hyperpycnites show uniform thickness in the study area, and may be correlated from place to place. The preliminary interpretation is that the CSM depositional environment was receiving clastics from the NE, which were primarily transported as density underflows (turbidites and hyperpycnites). However, significant storm deposits (tempestites) within the CSM indicate deposition occurred on a clastic marine shelf at water depths less than storm wave base.